Traces of Fukushima Radioactive material from the Fukushima nuclear accident is being transported across the Pacific Ocean by migratory fish, according to a new report.

Following the failure of coolant pumps at the Japanese nuclear reactor in March 2011, radioactive caesium (Cs) was released into the ocean in amounts that exceeded any previous accident, leading to considerable international concern about their spread.

Nuclear accidents are relatively infrequent, but have potentially wide-ranging effects on both human and ecosystem health.

Dr Daniel Madigan from Stanford University's Hopkins Marine Station says these effects are poorly understood, and that Fukushima presents an opportunity to better understand the dynamics and risks of radionuclide discharge into the ocean.

Migratory fish have long been thought likely ecological carriers of radioactive material around the oceans. Pacific bluefin tuna (Thunnus orientalis), also known as bluefin tuna, spawn in the western Pacific with some juveniles remaining in Japanese waters before migrating to the eastern Pacific at about one year of age.

The fish are harvested commercially for human consumption.

"We tested the possibility that juvenile [bluefin tuna] served as biological vectors of radionuclides between two distant ecoregions: the waters off Japan and the California Current Large Marine Ecosystem (CCLME)," he and colleagues write in the paper published today Proceedings of the National Academy of Sciences

"We analysed two-year-old [bluefin tuna] caught off San Diego, California in August 2011, known from size to be recent Japan migrants, for the presence of Fukushima-derived radionuclides."

They studied the tuna's white muscle tissue (where radioactive materials naturally accumulate) for the presence of radioactive caesium, as well as various naturally occurring radionuclides.

These results were compared with samples taken from California-caught fish from before the Fukushima incident, and from yellowfin tuna (T. albacares) which live predominantly in the CCLME.

They found elevated levels of Cs-134 and Cs-137 in all of the bluefin tuna caught after the Fukushima accident.

"In contrast," say the researchers, "[bluefin tuna] caught in 2008 and yellowfin tuna caught in 2011 had no measurable Cs-134, and consistently much lower Cs-137 concentrations."

"This is unequivocal evidence that Fukushima-derived radionuclides were transported to the CCLME by Pacific bluefin tuna as no other sources of Cs-134 were present in the North Pacific preceding the Fukushima disaster." Most other naturally occurring radionuclides were detectable only at very low concentrations.

Safety limits

The researchers found the combined Cs concentration in the fish caught in 2011 was up to ten times higher than in other years, but total radiocaesium concentrations were still significantly below the current Japanese safety limit.

Radioceasium levels also varied depending on the tuna's amount of exposure, their size and radioactive decay over time.

Food Standards Australia New Zealand (FSANZ), which monitors the situation in Japan in consultation with the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) say the levels in the fish are not of concern.

"The levels of Cs-137 (6.3+/- 1.4 Bq/kg) and Cs-134 (4.0+/- 1.5 Bq/kg) in Pacific bluefin tuna [in this report] do not represent any health concerns for Australian consumers," says a FSANZ spokesperson.

"Certain foods from Japan, including fish, are subject to testing at the Australian border prior to being allowed into the food supply."

"If the combined levels of C-137 and C-134 in fish are below 1000 Bq/kg the food is considered safe and allowed into the food supply."

"The 1000 Bq/kg limit is an internationally accepted safe level."

Leaving traces

Other highly migratory species such as seals, whales, billfish, turtles, sharks and seabirds that forage around Japan may also assimilate radiocaesium and transport it to distant parts of the North and South Pacific.

"Our results demonstrate that Fukushima-derived radionuclides in animal tissues can serve as tracers of both migration origin, and potentially timing, in mobile marine animals," say the researchers, "providing valuable complementary movement data to extensive tagging programs in the Pacific.

"[It is] an unexpected tool for examining migratory origins of apex predators in the Pacific Ocean."